Effects of exercise training and hypercholesterolemia on adenosine activation of voltage-dependent K+ channels in coronary arterioles

Chronic exercise mediates epigenetic suppression of L-type Ca2+ channel and BKCa channel in mesenteric arteries of hypertensive rats

BACKGROUND

Regular exercise is a lifestyle intervention for controlling hypertension and has an improving effect on vascular function. Voltage-gated L-type Ca (LTCC) and large-conductance Ca-activated K (BKCa) channels are two principal mediators of vascular smooth muscle cell contractility and arterial tone. The present study tested the hypothesis that DNA methylation dynamics plays a key role in exercise-induced reprogramming and downregulation of LTCC and BKCa channel in mesenteric arteries from spontaneously hypertensive rats (SHRs).

METHODS

SHRs and Wistar-Kyoto (WKY) rats were subjected to exercise training or kept sedentary, and vascular molecular and functional properties were evaluated.

RESULTS

Exercise inhibited hypertension-induced upregulation of LTCC and BKCa channel function in mesenteric arteries by repressing LTCC α1c and BKCa β1 subunit expression. In accordance, exercise triggered hypermethylation of α1c and β1 gene in SHR, with concomitant decreasing TET1, increasing DNMT1 and DNMT3b expression in mesenteric arteries, as well as altering peripheral α-KG and S-adenosylmethionine/ S-adenosylhomocysteine ratio. Acting synergistically, these exercise-induced functional and molecular amelioration could allow for attenuating hypertension-induced elevation in arterial blood pressure.

CONCLUSION

Our results indicate that exercise suppresses LTCC and BKCa channel function via hypermethylation of α1c and β1 subunits, which contributes to the restoration of mesenteric arterial function and vasodilation during hypertension.

Association of High Density Cholesterol With Hyperemic Epicardial Flow and Frame Count Reserve in Patients With Moderate Coronary Lesions and Slow Coronary Flow

Background

Patients with microvascular angina and non-obstructive coronary atherosclerotic disease have an elevated risk of adverse events and all-cause mortality compared with individuals without ischaemic heart disease. The diagnosis coronary microvascular dysfunction in this setting relies on the detection of impaired coronary flow at rest or on calculation of coronary flow reserve. Previous studies demonstrate that the coronary flow reserve assessed by the corrected thrombolysis in myocardial infarction method - the frame count reserve is an objective quantitative alternative to other widely used invasive methods for microvascular status evaluation.

Purpose

We assessed the significance of clinical, hemodynamic, angiographic variables and therapy with reference to FCR in a small group of patients with up to moderate atherosclerotic coronary lesions and slow coronary flow. Materials and methods: Frame count reserve was evaluated in 15 patients without flow-limiting (>50%) coronary stenoses admitted with unstable angina. Frame count reserve was calculated by dividing the baseline corrected thrombolysis in myocardial infarction frame count (cTFC) by the cTFC assessed after intracoronary infusion of 100 µg of the calcium channel blocker - verapamil.

Results

The values of frame count reserve correlate positively with the levels of high density cholesterol (r= 0.900, p=0.001), inversely coronary flow after the application of verapamil - cTFCv (r= - 0.534, p=0.049). cTFCv was positively related with the levels of high density lipoproteins (r = - 0.645; p= 0.044) and was negatively influenced by the presence of atherosclerotic lesions at quantitative angiography (42.8±19.1 (n=8) vs 23±5.4 (n=7), p=0.029).The therapy with β-blocker and long-acting nitrate was associated with insignificantly higher frame count reserves after intracoronary verapamil compared to the continuous intake only of β-blocker or β-blocker and verapamil (2.1±0.78 vs 1.34±0.14 vs 1.70±0.70, p=NS).

Conclusions

Higher high-density lipoproteins relate to higher frame count reserves evaluated using verapamil. The improved blood flow after this microvascular vasodilator is consistently positively related to high-density cholesterol and the lack of atherosclerosis at conventional coronary angiography. The combined intake of micro- and macrovascular vasodilator could be associated with higher frame count reserves compared to therapy with β-blocker and one vasodilating drug.

Cholesterol-Dependent Gating Effects on Ion Channels

Biomembranes separate a live cell from its environment and keep it in an off-equilibrium, steady state. They contain both phospholipids and nonphospholipids, depending on whether there are phosphate groups in the headgroup regions. Cholesterol (CHOL) is one type of nonphospholipids, and one of the most abundant lipid molecules in humans. Its content in plasma membranes and intracellular membranes varies and is tightly regulated. Voltage-gated ion channels are universally present in every cell and are fairly diversified in the eukaryotic domain of life. Our lipid-dependent gating hypothesis postulates that the controlled switch of the voltage-sensor domains (VSDs) in a voltage-gated potassium (Kv) channel between the "down" and the "up" state (gating) is sensitive to the ratio of phospholipids:nonphospholipids in the annular layer around the channel. High CHOL content is found to exert strong inhibitory effects on Kv channels. Such effects have been observed in in vitro membranes, cultured cells, and animal models for cholesterol metabolic defects. Thermodynamic analysis of the CHOL-dependent gating suggests that the inhibitory effects of CHOL result from collective interactions between annular CHOL molecules and the channel, which appear to be a more generic principle behind the CHOL effects on other ion channels and transporters. We will review the recent progress in the CHOL-dependent gating of voltage-gated ion channels, discuss the current technical limitations, and then expand briefly the learned principles to other ion channels that are known to be sensitive to the CHOL-channel interactions.

KV channels and the regulation of vascular smooth muscle tone

VSMCs in resistance arteries and arterioles express a diverse array of K_V channels with members of the K_V 1, K_V 2 and K_V 7 families being particularly important. Members of the K_V channel family: (i) are highly expressed in VSMCs; (ii) are active at the resting membrane potential of VSMCs in vivo (-45 to -30 mV); (iii) contribute to the negative feedback regulation of VSMC membrane potential and myogenic tone; (iv) are activated by cAMP-related vasodilators, hydrogen sulfide and hydrogen peroxide; (v) are inhibited by increases in intracellular Ca²⁺ and vasoconstrictors that signal through G_q -coupled receptors; (vi) are involved in the proliferative phenotype of VSMCs; and (vii) are modulated by diseases such as hypertension, obesity, the metabolic syndrome and diabetes. Thus, K_V channels participate in every aspect of the regulation of VSMC function in both health and disease.

Regulation of voltage-gated potassium channels in vascular smooth muscle during hypertension and metabolic disorders

Voltage-gated potassium (KV ) channels are key regulators of vascular smooth muscle contractility and vascular tone, and thus have major influence on the microcirculation. KV channels are important determinants of vascular smooth muscle membrane potential (Em ). A number of KV subunits are expressed in the plasma membrane of smooth muscle cells. Each subunit confers distinct kinetics and regulatory properties that allow for fine control of Em to orchestrate vascular tone. Modifications in KV subunit expression and/or channel activity can contribute to changes in vascular smooth muscle contractility in response to different stimuli and in diverse pathological conditions. Consistent with this, a number of studies suggest alterations in KV subunit expression and/or function as underlying contributing mechanisms for small resistance artery dysfunction in pathologies such as hypertension and metabolic disorders, including diabetes. Here, we review our current knowledge on the effects of these pathologies on KV channel expression and function in vascular smooth muscle cells, and the repercussions on (micro)vascular function.

Role of renal vascular potassium channels in physiology and pathophysiology

The control of renal vascular tone is important for the regulation of salt and water balance, blood pressure and the protection against damaging elevated glomerular pressure. The K⁺ conductance is a major factor in the regulation of the membrane potential (V_m ) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by V_m via its effect on the opening probability of voltage-operated Ca²⁺ channels (VOCC) in VSMC. When K⁺ conductance increases V_m becomes more negative and vasodilation follows, while deactivation of K⁺ channels leads to depolarization and vasoconstriction. K⁺ channels in EC indirectly participate in the control of vascular tone by endothelium-derived vasodilation. Therefore, by regulating the tone of renal resistance vessels, K⁺ channels have a potential role in the control of fluid homoeostasis and blood pressure as well as in the protection of the renal parenchyma. The main classes of K⁺ channels (calcium activated (K_Ca ), inward rectifier (K_ir ), voltage activated (K_v ) and ATP sensitive (K_ATP )) have been found in the renal vessels. In this review, we summarize results available in the literature and our own studies in the field. We compare the ambiguous in vitro and in vivo results. We discuss the role of single types of K⁺ channels and the integrated function of several classes. We also deal with the possible role of renal vascular K⁺ channels in the pathophysiology of hypertension, diabetes mellitus and sepsis.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Critical contribution of KV1 channels to the regulation of coronary blood flow

Ion channels in smooth muscle control coronary vascular tone, but the identity of the potassium channels involved requires further investigation. The purpose of this study was to evaluate the functional role of KV1 channels on porcine coronary blood flow using the selective antagonist correolide. KV1 channel gene transcripts were found in porcine coronary arteries, with KCNA5 (encoding KV1.5) being most abundant (P < 0.001). Immunohistochemical staining demonstrated KV1.5 protein in the vascular smooth muscle layer of both porcine and human coronary arteries, including microvessels. Whole-cell patch-clamp experiments demonstrated significant correolide-sensitive (1-10 µM) current in coronary smooth muscle. In vivo studies included direct intracoronary infusion of vehicle or correolide into a pressure-clamped left anterior descending artery of healthy swine (n = 5 in each group) with simultaneous measurement of coronary blood flow. Intracoronary correolide (~0.3-3 µM targeted plasma concentration) had no effect on heart rate or systemic pressure, but reduced coronary blood flow in a dose-dependent manner (P < 0.05). Dobutamine (0.3-10 µg/kg/min) elicited coronary metabolic vasodilation and intracoronary correolide (3 µM) significantly reduced coronary blood flow at any given level of myocardial oxygen consumption (P < 0.001). Coronary artery occlusions (15 s) elicited reactive hyperemia and correolide (3 µM) reduced the flow volume repayment by approximately 30 % (P < 0.05). Taken together, these data support a major role for KV1 channels in modulating baseline coronary vascular tone and, perhaps, vasodilation in response to increased metabolism and transient ischemia.

The Human Microcirculation: Regulation of Flow and Beyond

The microcirculation is responsible for orchestrating adjustments in vascular tone to match local tissue perfusion with oxygen demand. Beyond this metabolic dilation, the microvasculature plays a critical role in modulating vascular tone by endothelial release of an unusually diverse family of compounds including nitric oxide, other reactive oxygen species, and arachidonic acid metabolites. Animal models have provided excellent insight into mechanisms of vasoregulation in health and disease. However, there are unique aspects of the human microcirculation that serve as the focus of this review. The concept is put forth that vasculoparenchymal communication is multimodal, with vascular release of nitric oxide eliciting dilation and preserving normal parenchymal function by inhibiting inflammation and proliferation. Likewise, in disease or stress, endothelial release of reactive oxygen species mediates both dilation and parenchymal inflammation leading to cellular dysfunction, thrombosis, and fibrosis. Some pathways responsible for this stress-induced shift in mediator of vasodilation are proposed. This paradigm may help explain why microvascular dysfunction is such a powerful predictor of cardiovascular events and help identify new approaches to treatment and prevention.

Adenosine relaxation in isolated rat aortic rings and possible roles of smooth muscle Kv channels, KATP channels and A2a receptors

BACKGROUND

An area of ongoing controversy is the role adenosine to regulate vascular tone in conduit vessels that regulate compliance, and the role of nitric oxide (NO), potassium channels and receptor subtypes involved. The aim of our study was to investigate adenosine relaxation in rat thoracic aortic rings, and the effect of inhibitors of NO, prostanoids, Kv, KATP channels, and A2a and A2b receptors.

METHODS

Aortic rings were freshly harvested from adult male Sprague Dawley rats and equilibrated in an organ bath containing oxygenated, modified Krebs-Henseleit solution, 11 mM glucose, pH 7.4, 37 °C. Isolated rings were pre-contracted sub-maximally with 0.3 μM norepinephrine (NE), and the effect of increasing concentrations of adenosine (1 to 1000 μM) were examined. The drugs L-NAME, indomethacin, 4-aminopyridine (4-AP), glibenclamide, 5-hydroxydecanoate, ouabain, 8-(3-chlorostyryl) caffeine and PSB-0788 were examined in intact and denuded rings. Rings were tested for viability after each experiment.

RESULTS

Adenosine induced a dose-dependent, triphasic relaxation response, and the mechanical removal of the endothelium significantly deceased adenosine relaxation above 10 μM. Interestingly, endothelial removal significantly decreased the responsiveness (defined as % relaxation per μM adenosine) by two-thirds between 10 and 100 μM, but not in the lower (1-10 μM) or higher (>100 μM) ranges. In intact rings, L-NAME significantly reduced relaxation, but not indomethacin. Antagonists of voltage-dependent Kv (4-AP), sarcolemma KATP (glibenclamide) and mitochondrial KATP channels (5-HD) led to significant reductions in relaxation in both intact and denuded rings, with ouabain having little or no effect. Adenosine-induced relaxation appeared to involve the A2a receptor, but not the A2b subtype.

CONCLUSIONS

It was concluded that adenosine relaxation in NE-precontracted rat aortic rings was triphasic and endothelium-dependent above 10 μM, and relaxation involved endothelial nitric oxide (not prostanoids) and a complex interplay between smooth muscle A2a subtype and voltage-dependent Kv, SarcKATP and MitoKATP channels. The possible in vivo significance of the regulation of arterial compliance to left ventricular function coupling is discussed.

Exercise training enhances multiple mechanisms of relaxation in coronary arteries from ischemic hearts

Exercise training of coronary artery disease patients is of considerable interest, since it has been shown to improve vascular function and, thereby, enhance blood flow into compromised myocardial regions. However, the mechanisms underlying exercise-induced improvements in vascular function have not been fully elucidated. We tested the hypothesis that exercise training increases the contribution of multiple mediators to endothelium-dependent relaxation of coronary arteries in the underlying setting of chronic coronary artery occlusion. To induce gradual occlusion, an ameroid constrictor was placed around the proximal left circumflex coronary artery in Yucatan miniature swine. At 8 wk postoperatively, pigs were randomly assigned to sedentary or exercise (treadmill, 5 days/wk) regimens for 14 wk. Exercise training significantly enhanced the contribution of nitric oxide, prostanoids, and large-conductance Ca(2+)-dependent K(+) (BKCa) channels to endothelium-dependent, bradykinin-mediated relaxation in nonoccluded and collateral-dependent arteries. Combined nitric oxide synthase, prostanoid, and BKCa channel inhibition ablated the enhanced relaxation associated with exercise training. Exercise training significantly increased nitric oxide levels in response to bradykinin in endothelial cells isolated from nonoccluded and collateral-dependent arteries. Bradykinin treatment significantly increased PGI2 levels in all artery treatment groups and tended to be further enhanced after nitric oxide synthase inhibition in exercise-trained pigs. No differences were found in whole cell BKCa channel currents, BKCa channel protein levels, or arterial cyclic nucleotide levels. Although redundant, upregulation of parallel vasodilator pathways appears to contribute to enhanced endothelium-dependent relaxation, potentially providing a more refined control of blood flow after exercise training.

Exercise training-enhanced, endothelium-dependent dilation mediated by altered regulation of BK(Ca) channels in collateral-dependent porcine coronary arterioles

OBJECTIVE

Test the hypothesis that exercise training increases the contribution of BK(Ca) channels to endothelium-mediated dilation in coronary arterioles from collateral-dependent myocardial regions of chronically occluded pig hearts and may function downstream of H2O2.

METHODS

An ameroid constrictor was placed around the proximal left circumflex coronary artery to induce gradual occlusion in Yucatan miniature swine. Eight weeks postoperatively, pigs were randomly assigned to sedentary or exercise training (treadmill; 14 week) regimens.

RESULTS

Exercise training significantly enhanced bradykinin-mediated dilation in collateral-dependent arterioles (~125 μm diameter) compared with sedentary pigs. The BK(Ca) -channel blocker, iberiotoxin alone or in combination with the H2O2 scavenger, polyethylene glycol catalase, reversed exercise training-enhanced dilation in collateral-dependent arterioles. Iberiotoxin-sensitive whole-cell K+ currents (i.e., BK(Ca)-channel currents) were not different between smooth muscle cells of nonoccluded and collateral-dependent arterioles of sedentary and exercise trained groups.

CONCLUSIONS

These data provide evidence that BK(Ca)-channel activity contributes to exercise training-enhanced endothelium-dependent dilation in collateral-dependent coronary arterioles despite no change in smooth muscle BK(Ca)-channel current. Taken together, our findings suggest that a component of the bradykinin signaling pathway, which stimulates BK(Ca) channels, is enhanced by exercise training in collateral-dependent arterioles and suggest a potential role for H2O2 as the mediator.

Heart of the matter: coronary dysfunction in metabolic syndrome

Metabolic syndrome (MetS) is a collection of risk factors including obesity, dyslipidemia, insulin resistance/impaired glucose tolerance, and/or hypertension. The incidence of obesity has reached pandemic levels, as ~20-30% of adults in most developed countries can be classified as having MetS. This increased prevalence of MetS is critical as it is associated with a two-fold elevated risk for cardiovascular disease. Although the pathophysiology underlying this increase in disease has not been clearly defined, recent evidence indicates that alterations in the control of coronary blood flow could play an important role. The purpose of this review is to highlight current understanding of the effects of MetS on regulation of coronary blood flow and to outline the potential mechanisms involved. In particular, the role of neurohumoral modulation via sympathetic α-adrenoceptors and the renin-angiotensin-aldosterone system (RAAS) are explored. Alterations in the contribution of end-effector K(+), Ca(2+), and transient receptor potential (TRP) channels are also addressed. Finally, future perspectives and potential therapeutic targeting of the microcirculation in MetS are discussed. This article is part of a Special Issue entitled "Coronary Blood Flow".

Contribution of voltage-dependent K+ and Ca2+ channels to coronary pressure-flow autoregulation

The mechanisms responsible for coronary pressure-flow autoregulation, a critical physiologic phenomenon that maintains coronary blood flow relatively constant in the presence of changes in perfusion pressure, remain poorly understood. This investigation tested the hypothesis that voltage-sensitive K(+) (K(V)) and Ca(2+) (Ca(V)1.2) channels play a critical role in coronary pressure-flow autoregulation in vivo. Experiments were performed in open-chest, anesthetized Ossabaw swine during step changes in coronary perfusion pressure (CPP) from 40 to 140 mmHg before and during inhibition of K(V) channels with 4-aminopyridine (4AP, 0.3 mM, ic) or Ca(V)1.2 channels with diltiazem (10 μg/min, ic). 4AP significantly decreased vasodilatory responses to H(2)O(2) (0.3-10 μM, ic) and coronary flow at CPPs = 60-140 mmHg. This decrease in coronary flow was associated with diminished ventricular contractile function (dP/dT) and myocardial oxygen consumption. However, the overall sensitivity to changes in CPP from 60 to 100 mmHg (i.e. autoregulatory gain; Gc) was unaltered by 4-AP administration (Gc = 0.46 ± 0.11 control vs. 0.46 ± 0.06 4-AP). In contrast, inhibition of Ca(V)1.2 channels progressively increased coronary blood flow at CPPs > 80 mmHg and substantially diminished coronary Gc to -0.20 ± 0.11 (P < 0.01), with no effect on contractile function or oxygen consumption. Taken together, these findings demonstrate that (1) K(V) channels tonically contribute to the control of microvascular resistance over a wide range of CPPs, but do not contribute to coronary responses to changes in pressure; (2) progressive activation of Ca(V)1.2 channels with increases in CPP represents a critical mechanism of coronary pressure-flow autoregulation.

Effect of exercise training on nitric oxide and superoxide/H₂O₂ signaling pathways in collateral-dependent porcine coronary arterioles

Endothelial nitric oxide (NO) synthase (NOS) has been shown to contribute to enhanced vascular function after exercise training. Recent studies have revealed that relatively low concentrations of reactive oxygen species can contribute to endothelium-dependent vasodilation under physiological conditions. We tested the hypothesis that exercise training enhances endothelial function via endothelium-derived vasodilators, NO and superoxide/H(2)O(2), in the underlying setting of chronic coronary artery occlusion. An ameroid constrictor was placed around the proximal left circumflex coronary artery to induce gradual occlusion in Yucatan miniature swine. At 8 wk postoperatively, pigs were randomly assigned to sedentary (pen-confined) or exercise-training (treadmill-run: 5 days/wk for 14 wk) regimens. Exercise training significantly enhanced concentration-dependent, bradykinin-mediated dilation in cannulated collateral-dependent arterioles (∼130 μm diameter) compared with sedentary pigs. NOS inhibition reversed training-enhanced dilation at low bradykinin concentrations in collateral-dependent arterioles, although increased dilation persisted at higher bradykinin concentrations. Total and phosphorylated (Ser(1179)) endothelial NOS protein levels were significantly increased in arterioles from collateral-dependent compared with the nonoccluded region, independent of exercise. The H(2)O(2) scavenger polyethylene glycol-catalase abolished the training-enhanced bradykinin-mediated dilation in collateral-dependent arterioles; similar results were observed with the SOD inhibitor diethyldithiocarbamate. Fluorescence measures of bradykinin-stimulated H(2)O(2) levels were significantly increased by exercise training, independent of occlusion. The NADPH inhibitor apocynin significantly attenuated bradykinin-mediated dilation in arterioles of exercise-trained, but not sedentary, pigs and was associated with significantly increased protein levels of the NADPH subunit p67phox. These data provide evidence that, in addition to NO, the superoxide/H(2)O(2) signaling pathway significantly contributes to exercise training-enhanced endothelium-mediated dilation in collateral-dependent coronary arterioles.

Hypercholesterolemia increases mitochondrial oxidative stress and enhances the MPT response in the porcine myocardium: beneficial effects of chronic exercise

Hypercholesterolemia has been suggested to have direct negative effects on myocardial function due to increased reactive oxygen species (ROS) generation and increased myocyte death. Mitochondrial permeability transition (MPT) is a significant mediator of cell death, which is enhanced by ROS generation and attenuated by exercise training. The purpose of this study was to investigate the effect of hypercholesterolemia on the MPT response of cardiac mitochondria. We tested the hypothesis that familial hypercholesterolemic (FH) pigs would have an enhanced MPT response and that exercise training could reverse this phenotype. MPT was assessed by mitochondrial swelling in response to 10-100 μM Ca(2+). FH pigs did show an increased MPT response to Ca(2+) that was associated with decreases in the expression of the putative MPT pore components mitochondrial phosphate carrier (PiC) and cyclophilin-D (CypD). FH also caused increased oxidative stress, depicted by increased protein nitrotyrosylation, as well as decreased levels of reduced GSH in cardiac mitochondria. Expression of the mitochondrial antioxidant enzymes manganese superoxide dismutase (MnSOD), thioredoxin-2 (Trx2), and peroxiredoxin-3 (Prx3) was greatly reduced in the FH pigs. In contrast, cytosolic catalase expression and activity were increased. However, chronic exercise training was able to normalize the MPT response in FH pigs, reduce mitochondrial oxidative stress, and return MnSOD, Trx2, Prx3, and catalase expression/activities to normal. We conclude that FH reduces mitochondrial antioxidants, increases mitochondrial oxidative stress, and enhances the MPT response in the porcine myocardium, and that exercise training can reverse these detrimental alterations.

Contribution of voltage-dependent K⁺ channels to metabolic control of coronary blood flow

The purpose of this investigation was to test the hypothesis that K(V) channels contribute to metabolic control of coronary blood flow and that decreases in K(V) channel function and/or expression significantly attenuate myocardial oxygen supply-demand balance in the metabolic syndrome (MetS). Experiments were conducted in conscious, chronically instrumented Ossabaw swine fed either a normal maintenance diet or an excess calorie atherogenic diet that produces the clinical phenotype of early MetS. Data were obtained under resting conditions and during graded treadmill exercise before and after inhibition of K(V) channels with 4-aminopyridine (4-AP, 0.3mg/kg, iv). In lean-control swine, 4-AP reduced coronary blood flow ~15% at rest and ~20% during exercise. Inhibition of K(V) channels also increased aortic pressure (P<0.01) while reducing coronary venous PO(2) (P<0.01) at a given level of myocardial oxygen consumption (MVO(2)). Administration of 4-AP had no effect on coronary blood flow, aortic pressure, or coronary venous PO(2) in swine with MetS. The lack of response to 4-AP in MetS swine was associated with a ~20% reduction in coronary K(V) current (P<0.01) and decreased expression of K(V)1.5 channels in coronary arteries (P<0.01). Together, these data demonstrate that K(V) channels play an important role in balancing myocardial oxygen delivery with metabolism at rest and during exercise-induced increases in MVO(2). Our findings also indicate that decreases in K(V) channel current and expression contribute to impaired control of coronary blood flow in the MetS. This article is part of a Special Issue entitled "Coronary Blood Flow".

Metabolic syndrome reduces the contribution of K+ channels to ischemic coronary vasodilation

This investigation tested the hypothesis that metabolic syndrome decreases the relative contribution of specific K(+) channels to coronary reactive hyperemia. Ca(2+)-activated (BK(Ca)), voltage-activated (K(V)), and ATP-dependent (K(ATP)) K(+) channels were investigated. Studies were conducted in anesthetized miniature Ossabaw swine fed a normal maintenance diet (11% kcal from fat) or an excess calorie atherogenic diet (43% kcal from fat, 2% cholesterol, 20% kcal from fructose) for 20 wk. The latter diet induces metabolic syndrome, increasing body weight, fasting glucose, total cholesterol, and triglyceride levels. Ischemic vasodilation was determined by the coronary flow response to a 15-s occlusion before and after cumulative administration of antagonists for BK(Ca) (penitrem A; 10 microg/kg iv), K(V) (4-aminopyridine; 0.3 mg/kg iv) and K(ATP) (glibenclamide; 1 mg/kg iv) channels. Coronary reactive hyperemia was diminished by metabolic syndrome as the repayment of flow debt was reduced approximately 30% compared with lean swine. Inhibition of BK(Ca) channels had no effect on reactive hyperemia in either lean or metabolic syndrome swine. Subsequent inhibition of K(V) channels significantly reduced the repayment of flow debt ( approximately 25%) in both lean and metabolic syndrome swine. Additional blockade of K(ATP) channels further diminished ( approximately 45%) the repayment of flow debt in lean but not metabolic syndrome swine. These data indicate that the metabolic syndrome impairs coronary vasodilation in response to cardiac ischemia via reductions in the contribution of K(+) channels to reactive hyperemia.

Role of potassium channels in coronary vasodilation

K+ channels in coronary arterial smooth muscle cells (CASMC) determine the resting membrane potential ( Em) and serve as targets of endogenous and therapeutic vasodilators. Em in CASMC is in the voltage range for activation of L-type Ca2+ channels; therefore, when K+ channel activity changes, Ca2+ influx and arterial tone change. This is why both Ca2+ channel blockers and K+ channel openers have such profound effects on coronary blood flow; the former directly inhibits Ca2+ influx through L-type Ca2+ channels, while the latter indirectly inhibits Ca2+ influx by hyperpolarizing Em and reducing Ca2+ channel activity. K+ channels in CASMC play important roles in vasodilation to endothelial, ischemic and metabolic stimuli. The purpose of this article is to review the types of K+ channels expressed in CASMC, discuss the regulation of their activity by physiological mechanisms and examine impairments related to cardiovascular disease.